1. Field of the Invention
The present invention relates to a halftone image generation method of generating a halftone image formed on a printing plate for letterpress printing, a threshold matrix which is compared with a grayscale original image in generating a halftone image, and a threshold matrix generation method. The present invention is also relates to a printing plate for letterpress printing and a printing plate manufacturing method.
2. Description of the Background Art
Conventionally, letterpress printing such as flexography has been performed in commercial printing, and a printing plate for letterpress printing is manufactured by using a technique of CTP (computer to plate) or the like. In manufacturing the printing plate for letterpress printing, for example, laser beams are applied (irradiated) to a mask layer laminated on a photosensitive resin layer by using the CTP, and openings corresponding to a halftone image generated from a grayscale (i.e., continuous tone) original image are formed in the mask layer. UV (ultraviolet) light is applied (irradiated) to the photosensitive resin layer through the openings formed in the mask layer to cure exposed portions of the photosensitive resin layer, and then a development process of removing the mask layer and non-cured portions of the photosensitive resin layer is performed to form a printing plate where convex portions corresponding to the halftone image are formed on its surface.
To create a halftone image for printing from a grayscale original image, for example, an AM (Amplitude Modulated) screening where gray levels are represented by changing the size of dots which are regularly arranged (accurately, the halftone dots are clusters each of which is a group of dots (or pixels) connecting one another), has been used. Actually, a threshold matrix in which a plurality of elements are arranged in a row direction and a column direction and a threshold value is assigned to each element, is prepared and generated in advance, and a halftone image used in printing is generated by comparing the original image with the threshold matrix.
In the letterpress printing, in a case where the size of minimum halftone dots is small or the screen ruling of halftone dots is small (i.e., the pitch of halftone dots is large), reliefs formed on a printing plate become narrow or small in a highlight area having a small dot area rate (i.e., a low density of halftone dots) in the printing plate. This results in causing failure in the formation of reliefs where the reliefs are bended by lack of the strength of reliefs in the development process in manufacturing of the printing plate or the like, and results in reducing reproduction in printing for appropriately representing the original image in a printed image. Reproduction in printing may be reduced with the reliefs being bended by pressure in printing.
The phenomenon that the reliefs become small in the highlight area can be solved by making the size of minimum halftone dots larger than the size of pixels in the minimum resolution by using a threshold matrix shown in FIG. 6.24 of pp. 168 to 170 of “Fundamentals of Electronic Imaging Systems” by F. W. Schreiber, U.S.A. the third edition, Springer-Verlag New York, May 1, 1993 (Document 1).
U.S. Pat. No. 7,126,724 (Document 2), U.S. Pat. No. 7,245,402 (Document 3), European Patent Application Laid-Open No. 1,557,279 (Document 4), and European Patent Application Laid-Open No. 1,684,499 (Document 5) disclose a technique for increasing the strength of reliefs where dots for reinforcing a relief are formed at a position apart from the relief. The dots for reinforcing are convex portions which are lower than the relief for printing and not included in a target for printing. US Patent Application Publication No. 2003/48477 (Document 6) discloses a technique for changing the shape of each halftone dot into a cross between the highlight area and an area of middle gray levels (i.e., in an area between the highlight area and a shadow area having a large dot area rate).
If the size of minimum halftones dots is increased as disclosed in Document 1, since the number of halftone dots included in the highlight area is reduced to maintain a desired dot area rate, the pitch of halftone dots becomes large and the strength of reliefs is decreased. As a result, reproduction in printing is reduced.
In Documents 2 to 5, although the dots for reinforcing are not included in a target for printing, there is a possibility that the height of dots for reinforcing becomes equal to that of the reliefs for printing in such a case where intensity of light which is applied to portions to be dots for reinforcing is larger than a desired intensity in forming a printing plate, and the dots for reinforcing which should not be normally printed are printed (so-called scumming occurs). On the other hand, in a case where the intensity of light which is applied to the portions to be dots for reinforcing is smaller than the desired intensity in forming the printing plate, the dots for reinforcing become smaller than necessary and the reliefs are not fully reinforced, and thereby the strength of reliefs may be decreased. In both cases, reproduction in printing is reduced.
In Document 6, since the shape of halftone dot is changed into a cross also in the area of middle gray levels, the length of circumference of halftone dot is longer. Therefore, a printed dot becomes thicker or thinner by a slight change of printing pressure in printing, and reproduction in printing is reduced.